Bacterial communities in water cooling systems treated with bactericides often become resistant to these bactericides. This has been ascribed to selection for resistant cells. Certain bacteria, having a high inherent susceptibility to water treatment bactericides become dominant in systems after bactericide treatment. We investigated the idea that bacterial isolates adapt to grow in the presence of bactericides. Pure cultures of Pseudomonas aeruginosa, P stutzeri and Bacillus cereus were cultured repeatedly in the presence of sub-inhibitory concentrations of 2,2'-methylenebis(4-chlorophenol), Na dimethyldithiocarbamate, isothiazolone and alkyl dimethyl ammonium chloride. All isolates adapted to grow in the presence of increasing concentrations of the bactericides. The phenomenon of development of bacterial resistance to water treatment bactericides was ascribed to adaptation and not to selection.